I don't think your bank analogy is very strong, but never mind that.
I agree with what you're saying in principle, that if a user/customer
buys bit delivery at a fixed rate then we should deliver it.
But isn't that the point. You can't guarantee delivery, just as you
can't guarantee you won't get a busy signal when you make a call.
But if the carrier tunes their network so you will never get a busy
signal when calling into 900 numbers from which they receive a
kickback (hosted on their network or just "preferred partners"), at
the cost of a greater likelihood of busy signals for calls which are
not as profitable for them, this is "enforcing the provider's
preferences and not the customers".
When carriers start to tune their network so not only do VOIP
connections to their own servers get a higher QoS, but also in a
manner which tends to *induce* jitter and other 'Q'uality degradation
for Skype and Vonage, then it's time for them to lose "common carrier"